This application is based on application No. 10-214631 filed in Japan, the contents of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a technique for improving, in particular, the gradation reproduction of an image in an image forming apparatus comprising an optical beam scanner.
2. Description of the Related Art
To respond to recent requests for color printing, systematization, and the like made to electrophotographic image forming apparatus, there has widely been used a digital image forming apparatus which forms an image by driving a laser diode to scan-expose a photosensitive drum based on the gradation value of image data and developing an electrostatic latent image formed thereby with a toner.
In such a digital image forming apparatus, the intensity of a light beam emitted from a laser diode or a time during which the light beam is emitted from the laser diode is modulated based on the gradation value of image data to form an electrostatic latent image by exposing, to a spot beam, each pixel on the photosensitive surface of a photosensitive drum. The electrostatic latent image is changed to a visible image by means of a developing device, thereby forming a toner image. The toner image formed on the photosensitive drum is transferred onto a recording sheet by the electrostatic force of a transfer charger and further fixed with a fixing device, resulting in an image.
The amount of toner adhered to one dot varies with a variation in the beam intensity or light emitting time of the laser diode, which allows the gradation of an original image to be represented.
In such an electrophotographic image forming apparatus, however, it is difficult to establish a completely linear relationship (linear proportion) between the density of a reproduced image (development density) and the density of the original image due to the development properties of a toner, the exposure properties of a photosensitive member, or the like. Gradation reproduction is particularly poor in a high density portion and a low density portion.
Specifically, the original density and the development density have a substantially linear relationship in a middle density portion, while having a non-linear relationship in high and low density portions where the development density changes at a lower rate than the original density.
To reproduce variations in the gradation of the original image with fidelity, it will easily be understood that a linear relationship is established preferably in any density region of the reproduced gradation.
On the other hand, there has been the problem that an image of continuous dots and an image of discontinuous dots have difference densities even if they are developed and formed based on density data representing the same gradation value, so that the reproduced gradation differs depending on a pattern to be reproduced.
The reason for this may be that, even though an exposed region of a single beam spot on the photosensitive surface is undevelopable, when continuous dots are formed, the undevelopable exposed regions of the individual beam spots overlap each other so that a developable exposure intensity is reached and a larger amount of toner is adhered thereto. Accordingly, the density is higher in the case of plotting continuous dots than in the case of plotting one discontinuous dot.
In an image forming apparatus which reproduces an image based on halftone image data, the gradation is normally represented by varying the density of halftone dots (the number of halftone dots per unit area). However, it is difficult to represent subtle gradation variations only by varying the density of halftone dots and a desired gradation can not be obtained.
The present invention has been achieved in view of the foregoing problems. It is therefore an object of the present invention to provide an image forming apparatus capable of forming an image with excellent gradation reproduction.
To attain the above object, one aspect of the present invention is An image forming apparatus for forming an image having a gradation based on image data, comprising: a light beam generating element capable of independently changing a beam output and a beam diameter; a scanning unit which performs scanning with the beam generated from the light beam generating element; a first controller which controls the beam output based on a gradation level of the image data; and a second controller which controls the beam diameter based on the gradation level of the image data.
Another aspect of the present invention is an image forming apparatus for forming an image having a gradation based on image data, comprising: a light beam generating element capable of independently changing a beam output and a beam diameter; a scanning unit which performs scanning with the beam generated from the light beam generating element; a first controller which controls the beam output based on a gradation level of the image data; and a second controller which controls the beam diameter so that the beam diameter in forming a discontinuous dot is larger than that in forming continuous dots.
Still another aspect of the present invention is an image forming apparatus for forming an image having a gradation based on image data, comprising: a light beam generator capable of independently changing a beam output and a beam diameter; a scanning unit which performs scanning with the beam generated from the light beam generating element; a first controller which controls the beam output based on a gradation level of the image data; a text region judging unit which judges whether or not the image is a text image based on the image data; and a second controller which controls the beam diameter so that the beam diameter in forming a text image is larger than that in forming a non-text image.
Yet another aspect of the present invention is an image forming apparatus for forming an image having a gradation on a image carrying member based on image data, comprising: a light beam generating element capable of independently changing a beam output and a beam diameter; a scanning unit which performs scanning with the beam generated from the light beam generating element; a first controller which controls a light emitting time of the light beam generating element per dot based on a gradation level of the image data; and a second controller which controls the beam diameter so that an area having exposure energy in excess of a specified threshold value on the image carrying member is linearly proportional to the length of the light emitting time.
These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate specific embodiments of the invention.